Horned animal
(Other definitions for ram that I've seen before include "Thrust" , "Aggressive sheep?" , "Butt" , "An animal; forcibly crash into" , "The constellation Aries" .)
(Other definitions for gnu that I've seen before include "Gun to get antelope" , "animals" , "Animal" , "Gun for a kind of deer" , "hitherto unknown, we hear" .)
(Other definitions for wildebeest that I've seen before include "Gnu" , "game in Africa" , "antelope" , "beast" , "large animal" .)
'horned animal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rhino that I've seen before include "This beast may make a charge" , "African/Asian ungulate" , "Stumpy" , "Dosh (slang)" , "Horned pachyderm" .)